Weird Install from previous hosting

I just switched hosting companies. Today is day 11 that my site has been down. My new hosting company is saying it’s due to a weird install from my previous hosting. There are html and database codes that are wrong. They can’t tell WHAT is wrong with them, they just told me to go here. I’m clueless when it comes to codes, websites and such. I don’t know where to start or what I am asking, to be honest. I just want my site to run.