Sqaure boxes replacing all my texts.
 

I am a very begineer to ubuntu 18.04 . I recently downloaded 5 fonts from the internet and installed all of them . The system didn't warn me any thing.When I turned on my laptop after few hours and logged into my account all my texts were replaced by square boxes and except google chrome which was not my default browser was replaced by square boxes. Terminal also had square boxes. Mozilla didn't open. Neither was my new app that I installed was opening. Please help me!! I dont have dual OS and the laptop which I bought recently is facing this problem so please help me urgent

Which fonts did you install, and what did you do to install them?
Does this mean that your login screen is still working as expected? Only after you entered your user name and password, you get the problem?
I can imagine a few solutions or workarounds. First, is there a way to change your desktop environment at the login screen? Or to select some sort of fall-back desktop? I would try that and uninstall those fonts.

In case this doesn't work, enter a non-graphical environment. If you can log on to the PC via the network, from a different device, try that. Several apps exist for Android and Apple devices that allow you to use SSH to enter your PC. If you have a Windows PC, try Putty.

Or, on the login screen, hit ALT-F1, ALT-F2 etc. until you get a text-based login prompt. I mean press the ALT key and the F1 key at the same time. You may have to press CTL as well. You can then login and uninstall the fonts, but since you are a newbie, this requires more handholding.

Or you recover your data and reinstall.

Or if you have an installation DVD, boot from it and enter rescue mode. However, again, this requires more handholding.

You could log on to non-graphical mode. Normally, when you see the login

It's not really possible to help unless we know
- your distro
- how you installed the fonts