Exported video stops at 1 min

Here's my issue. this only started within the last two weeks and the adobe chat has been no help - and has in fact wrecked my computer further since they have remotely controlled it.

Each time I try to export a video in h.264 at the youtube 1080p setting it will either export and look totally fine, no errors, then when i go to open it it won't play.

OR it will again export and look fine and stop playback around two minutes.

I have ensure the entire sequence is selected, I have uninstalled and reinstalled it. I have literally spent my entire weekend on adobe's remote support chat trying to work this out & now they are just saying they are of no help.

Prior to this the remote chat support went into my computer & changed a bunch of stuff, including setting up a new administrator account - now even in the admin account I can't export to my external hard drive (I was previously doing this) and when I asked them how this had occurred they said, 'you can export to a hard drive' - obviously untrue. I also can't save to my external, and they've done something weird to my rubbish bin which is showing up as an error EVERY ten minutes on my computer.