are you on the trial plan?

yes

you have very likely ran out of disk space on the 500mb volume, please upgrade to hobby for access to 5gb volumes, once you upgrade go into the volumes settings and grow the volume to 5gb

Is there a way to see how much disk I have used?

you can click on the volume

keep in mind it might not show you that you have hit the limit, but the crash is caused by mongo trying to allocate more storage than what you have free