Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I suspect the screen or LCD screen itself, is what has gone bad.
Does your camera have a smaller screen higher up? if so, that's a viewfinder. What does it do?
How old is the camera? in months or years since you bought it.
Has the camera been dropped or otherwise subjected to mechanical shock(s) in its lifetime?
Has the camera been operated in or subjected to, temperatures below 32 degrees F (freezing point) or above about 100 degrees F ?
Has it ever been subjected to a humidity higher than 85 % or gotten wet ?
Has the camera been operated in a gritty or sandy environment, such as a beach, a seaside, a construction site, or the like?
Then, given that it was disassembled, you may have not quite placed back properly, a delicate cable (called a 'ribbon cable' because it looks like ribbon) going to the main LCD screen. Or may have dislodged it ever-so-slightly.
You will need to disassemble it again, and carefully seat the ribbon cable to the LCD screen properly this time, and without pulling it loose while putting the case back on.
(And no, if the screen was working when you powered it up before reassembling it, then that *didn't* cause the problem, at all... in fact, it was rather good-enough procedure.)
Let me know if that answers your question in full, or if you need more advice instead.
Then it may simply be in a state of shock - esp. if (perhaps) you did not observe sufficient anti-static precautions while handling components with the camera opened.
I'd say, close it up, remove the battery, wait with it like that overnight, put the battery back in, and see whether that (rest and reset) gets it working again. It might.
Then it may just have to be sent to a repair facility, or to other than Canon, for repairs.
Let me know if you would like an estimate on non-Canon repair... I can provide one.
I have no idea, frankly. Such knowledge is usually held proprietary by Canon, regrettably.
A camera shop might be able to provide either piece of info, though if they only provide the part # to you, I could try to find a source by the part #.
Can you see a part # on it? or anything that might be one?
It is late (in my time zone) and it would be advisable for me to quit and try to find that tomorrow morning, with my full faculties in order. Is that OK?
(That does sound like a part #.)
I cannot find information on that part #, I'm sorry to have to report.
Is it possibly attached permanently at one end? if so, to what part? (I don't need a part # for that other part, just a vague identification... if this is actually how it is, that the cable is not an independent part.)
But what you have is a TFT-M... are you sure it isn't the wrong part # for this part? I'm concerned lest there be a mistake, perhaps.
OK. But I would be cautious about expectations.