Have you tried rebooting your machine?
Any idea of what changed between it working and now? OS update or anything? Install any new hardware?
Did any codec packs get installed like K-Lite? Back in the day, some of those third-party codecs would really mess with AE.
Try going to the edit menu and select the output module template to see if anything opens. If nothing happens then try rebooting then re-launch After Effects holding down the Shift + Control/command or alt/option keys (if I remember correctly) to reset your preferences. Could also use references panel and browse to your preferences file then rename it and reboot and relaunch.
It was as simple as that!
Thank you very much, Rick.
Thanks for your reaction!
Needed to reset my preferences.