One thing does nt seem to have been checked - Can you enter Safe Mode?
To enter Safe Mode
As the POST screen ends repeatedly tap the F8 key until you get a selection screen - choose one of the Safe mode Options. This may take a few attempts to get the timing right so please persevere
If you can enter safe mode you should be able to access your important files and back them up.
If you cannot access safe mode there are other more complex methods that we can suggest but the thing here is to try things calmly step by step