Picture doesn't fit the screen

Without seeing the dark cycle myself it is difficult to guess what. I only can think of something hit the screen (ball?) or perhaps someone bumped into the screen?
If the bump was minor, it could become lesser.
Perhaps you should ask a technician to look, or take a picture from the screen, one when switched on and one switched off, but close to where the dark circle is and take the pictures to a tech.

Can you include a picture of the screen here; Johson? If so, it will be more helpful to provide a solution to this problem. If you wish to get some details; check the site linked here. Viewing it in "Mosaic" or "Magazine" will make surf easy. Pull up older posts. Surf the site with patience. Do a search by type in the brand name of your device at its search box and press "Enter".
http://electronicshelponline.blogspot.com/
Screen-shots to defective LCD are given. You can see and compare to that to your TVs', and can make sure about the fault to an extent.

they have given you another faulty screen.

Yes.... u must adjust the screen voltage going to picture tube....located on the CRT module>>>little board connected directly to picture tube (take back off TV and you will see ) play around with screen control on the CRT module......can be adjusted at times to keep from losing picture!!! tube is bad...i know sure looks good when showing a picture....TRY adjusting screen voltage ( YES while TV is ON>>>CAREFULL ) gl

Sometimes the box that sends the signal to the TV preconditions the signal to 4x3 or 16x9, depending on the capability of the TV. I always opt to the "no fit" option and allow the intelligence of the TV to fit the screen.

If you have "off air" TV signal without that preconditioning, I cant help.

Adjust the Aspect ratio. The manual will tell you how.

if you have cable box or a sattelite change the resolution of the STB to 4.3

Hello! I have a 32" Vizio LCD...within 30 minutes of the tv being turned on, black lines start running through the bottom half of the screen and eventually blacking out the bottom half of the tv screen. It also doesn't have anything to do with the ratio as I have tried that. HELP!

look for horizontal control

Pls more info; what do you mean tv on, picture froze? you are watching on tv chanel, do you use cable? I can help you but need more details.