A picture of the image produced, even taken with a cell phone would help. But there are really two likely possibilities here.
1. Part of the camera structure has come away and is obscuring part of the microbolometer window it could also be the FFC flag not sitting in the correct position at rest.
2. The microbolometer is damaged due to an electronic failure or a fall causing mechanical damage to the die.
Do you know the history of the camera ? Was it used at any time on an airborne vehicle ?
Sadly damage to the microbolometer is not repairable if the FFC cannot cope with it.
A quick check worth doing, as it is non invasive, is to undo the lens lock screw and remove the lens. If it is a screw fit, count the turns using a reference malt so you can refocus the lens easily. If it is a slide mount, just mark the lens position. You will then be able to see the microbolometer and FFC flag operating. Is the FFC flag incorrectly positions obscuring the microbolometer ? There should not be anything obscuring the microbolometer window.
A picture of what you see with the lens revised would be good for me to assess.
Fraser