Toshiba 55LF621U19 fires up and freezes at the the Prime home screen

It's possible that the 5V2 rail on your Toshiba TV power supply board is not being used, and that's why you're only seeing voltage on the 5V1 rail. However, it's also possible that the power supply board is faulty and not providing voltage to the 5V2 rail.
To determine if the power supply board is faulty, you can use a multimeter to measure the voltage at the connector for the 5V2 rail. If you're not getting any voltage there, it could indicate a problem with the power supply board. However, if you do see voltage on the 5V2 rail, it's likely that it's just not being used.
You can consult your TV's user manual or contact Toshiba's customer support for more information about the purpose of the 5V2 rail on your TV's power supply board.

You can find used power supply boards and main boards on ebay, maybe other sites that offer repair parts, but it's hard to say if the damage was limited to just the power board. You might get lucky and fix it with a replacement board, or you could end up several hundred dollars more to keep the TV working. You just have to decide if it's worth the investment to fix vs replace.

Total right off unfortunately. 125,000 volts is very damaging to all the components. Claim on house insurance, some cover it.

this is a PC >? no it is a television
so will move you to there
no model TV told see rear of the model is there
im sure the TV settings are wrong for you
get the manual from make, OEM TOSHIBA to learn settings.

Is the TV totally dead or do you have a standby light?

Prime Video is available through your computer's web browser as well as hundreds of streaming media devices, including:

• Smart TVs
• Blu-ray players
• Set-top boxes (Roku, Google TV, TiVo, Nvidia Shield)
• Amazon Fire TV
• Fire TV Stick
• Game consoles (PlayStation, Xbox, Wii)
• Android devices (Prime Video app for Android phones and tablets)
• iOS devices (Prime Video app for iPhone, iPad & iPod touch)
• Fire phone
• Fire tablets

https://www.amazon.com/gp/help/customer/display.html?nodeId=201422820

1. Plug your Firestick into one of the HDMI ports on your TV. Plug the USB power cable into your Firestick and into the wall or a USB outlet on your TV (if there is one)
2. Turn on your TV and set the INPUT device to the correct HDMI port. There should be a splash screen or setup screen on your TV at that point.
3. Using your Firestick remote, navigate to network settings and scan for your WI-FI router, Then enter the Wi-Fi password for your router (the same one you use for your smart phone or laptop).

You should be able to fully use your Amazon Firestick, and if you have Amazon Prime, be able to access all prime and free offerings from Amazon.

Just like Netflix, Amazon has Prime Instant Video apps available for all manner of connected TVs, Blu-ray players, cinema systems and games consoles, meaning whatever home setup you have, you should be covered.
The platform is compatible with more than 400 devices in total, a full list of which can be found in Amazon's support pages. If you've bought a connected LG, Samsung, Sony or Panasonic TV in the last few years, however, chances are you're covered.
The app can be downloaded - for free - from your TV's respective app store. Once you've signed in with your membership details that's it, you're set and ready to enjoy some streaming action.
Getting into the Amazon Prime way of life is just as straightforward for console owners, with dedicated apps available for PS3, PS4, Xbox 360 and Xbox One. Even those with a Wii or Wii U still sat under their TV can get in on the action.
How to watch Amazon Prime without a Smart TV
If your home entertainment system is a little out of date and in need of some serious sprucing, fret not, Amazon has got your back with a couple of affordable own-brand, internet-connected hardware additions to give you instant Prime access.
Amazon Fire TV is a £79 set-top box turned micro-console which lets you both live stream and enjoy a bit of big screen app-based gaming. By contrast, the £35 Fire TV Stick is a stripped-back answer to Google Chromecast.
Pleasingly, both come bundled with a free 30-day Prime trial and if you plump for either, you're not going to be locked down to just Amazon's content services. The connected tools let you download and enjoy services such as Netflix, BBC iPlayer, vevo and, if you're really desperate for something to watch, Demand 5.
http://electronicshelponline.blogspot.com/

I would run a good virus or malware program, there are some very good and free ones available. Try Malwarebytes, AVG and Spybot S&D, all available free and all very good. Hope this helps.