Power surged at a gig last night amp would power up but hummed .now it wont power up at all

A silly suggestion is to plug in a different guitar and see if the problem still happens. Have you checked the link your using to connect to the amp? For example is it a faulty lead, a loose jack plug on the end of the lead or an intermittent signal from a wireless connection? You could also check the pickups on the actual bass guitar.
Pick ups don't often cause issues but they can become faulty because of age, corrosion, short circuits and if they're badly fitted.

Boy, there are a bunch of things which can cause that, Jammy. First of all, is the wiring in your home, or at gigs or wherever providing an adequate ground ( I assume you have a 3 pronged plug to your amp). Does it do this everywhere? If it does,you've likely got an internal problem in the amp. i can't help you with that. There are too many possibilities. Is this a tube amp ? If it is ( I kinda think all Marshalls are), then I have no idea. I realize this is not answering your question, but the causes of the problem are vast. Take it in to a reputable guitar shop. That's my answer. . .

Why was it on for over a year :D Has anyone replaced the fuse? In the service manual I found it has to be only 2 Amps! Okay let's solve the problem. The power transformer was overloaded which caused the broken winding. It is still overloaded what can be heard by the hum sound. First thought was to check main capacitors or better just replace them! If the hum gets louder, they might also become warm. Check voltages after bridge rectifier and smoothing circuit. They should be +/-42 V (TP3, TP4) and +/- 16 V (TP5, TP6) Let me know which of them is faulty Cheers

Likely the filter capacitors have a problem. Best to take this in for repair... it is ODD that there is noise with the standby switch is in standby... That makes no sense... In standby it appears that power is removed from the power amp high voltage so nothing should come through the speaker... Unles maybe one off the power tubes had a total meltdown which should have blown the fuse.

Either the speaker or the amp in it or both fried. Check to see IF your warranty will cover the repair.
Make sure that in your venues ALL interconnected equipment is powered from teh same receptacle OR power conditioner module, incling mixers, amps, speakers. Ground bounceof wiring in building power that causes damage most likely will not be covered under warranty.

The amp has a failure. One side of the power amp is starting to draw excessive current to the point that the filter caps can't sustain the current so the ripple starts to be heard in the speaker. Continued use MAY burn up the speaker by sending DC through it as well as causing additional damage to circuit boards, etc..

Probably the power amp chip has failed with a short to one of the power rails. This will cause hum and ultimately wreck your speaker. Take in for repair.

The unit will require repair and if you are not adept at electronics, you will need to take it to a shop. Likely there is failure in the power supply or amplifier that has blown the fuse. Replacing the fuse WITHOUT repairing the cause MAY do additional damage.

Try disconnecting the speaker from the amp with the amp unplugged from your power. Never work on an amp when its plugged in unless you know how dangerous it is. Check for resistance across the speaker terminals with a volt ohm meter to see if you have a blown speaker. Thats the first thing to look for. You should here a hiss or a low hum when your amp is on and you plug a guitar cord into the input without the guitar. Check those two things first.

The power supply is located in that area on one side. It is possible that one of the output op-amp ICs has failed. There are a number of 4558 ICs used as output buffers for this unit. Without seeing the unit and testing parts of it, I can't say for sure. These are not the easiest to work on as they are really compact. Where are you located? I may be able to suggest a service center in your area. The repair cost should not be too bad. The parts are cheap and readily available. Please update this with your location. Thanks, Dan