First of all STAY CALM!
Don't do anything rash and take a slow methodical approach
+1 to checking cables etc.
I would also try saving and turning your PC completely off and then on again (a very IT "fix all" solution and always worth a try)
Another cause of crackles and pops in audio within a DAW is having the audio buffer setting in your sound card driver set too low for what you are doing
Typically you would set them low for recording to reduce latency
But when you start adding fx during the mixing process you add more strain onto your PC and it struggles to play the audio back with small buffer settings, and you end up with crackles and pops
Fortunately, when mixing latency doesn't matter so you can relieve this strain by increasing your buffer settings - this should hopefully resolve your issue
This should help you find the settings:
https://www.steinberg.net/nc/en/support/knowledgebase_new/show_details/kb_show/the-playback-of-my-song-crackling.htmlIf this doesn't work please post as much info as you can and we can try and track the issue down