Hi ! welcome to the forum. /We are all here to help/
This can't be too serious of a problem... sooooo..
see if this command (from the command line) fixes your source list.
if not, advise us of the errors you get ....and
sudo apt-get update
post these results
to see what release you are running
to see what you source list is. The latter command will output the text file sources.list.txt in the your current working directory////cut and paste it between code tags (# at top of post screen)....
cat /etc/apt/sources.list > sources.list.txt
we await your reply <==BDQ