first, relax. It is likely an easy fix, and if not it is likely easy to reinstall everything from the ground up, and likely your friend that lent you their laptop won't mind any of this.
I would start by asking your friend for help. it may be embarrassing, but I assure you I would rather you came to me for help than tried to hide it.
If you still want to work on this yourself, /join your distro's IRC channel where you can chat with people who might walk you through fixing it. warning, they may make things worse too, not intentionally, but online help can be tricky.
There are many ways to connect to IRC. I now recomend the extra steps of signing up for Matrix and then using the bridge because it works much better if you also want to use it on a phone, which can be very handy for taking a picture of your screen when it goes into 'grub-repair'.
best instructions I can offer will get you to PS1's chat
where we may be able to help you, but there are only 2 or 3, as where #ubuntu has over 1300