All of a sudden, my LG GH22LP20 stopped recognizing cd's?

thats strange usually if it reads and burns dvd's it should be the same with cd's

Click start control panel,administrive tools,computer management,device manager scroll to cd/dvd rom drives right click properties then properties again then tick enable cd digital cd audio for this cd rom device or scroll to dvd regions select the no _ for your specific area
If this does not fix the problem you might try
you might have to physically unplug the leads for this cd-rom then plug them back in this should activate the found new hardware wizard or
you might have to uninstall then reinstall your DVD/CD or uninstall the driver then reinstall for this device
click start control panel admistrive tools,computer management,device manager scroll down cd dvd rom drives right click you will see options to update driver,roll back driver,or
select properties general troubleshoot, select driver click to reinstall drivers click update driver or uninstall then reinstall
make sure that the cd-dvd burner is getting enough power test all leads that are attached to the dvd drive including electrical extensions and make sure they have a secure connection or just replace them there might be bad/faulty or

if this does not fix the problem it might be a registry problem
Close all open programs

Click on Start, Run, and type regedit and press Enter
Click on the plus signs (+) next to the following folders

HKEY_LOCAL_MACHINE

SYSTEM

CurrentControlSet

Control

Class

{4D36E965-E325-11CE-BFC1-08002BE10318}

This folder is the DVD/CD-ROM Drive Class Description in the registry. Look for any of the following names in the right hand column.

Upper Filters

Lower Filters

Upper Filters.bak

Lower Filters.bak
If any of the above keys shown in step 4 are listed, right-click on them and choose Delete

After deleting the keys, close the registry editor
Reboot your computer
Open My Computer and check to see if your CD or DVD drives have returned. You may also want to open Device Manager and verify that the yellow exclamation and error code on the CD or DVD drive is gone.