Hi everybody.
Because there are some general advices to fix troubles I decided to collect it all at one place.
- Try to do integrity check: admin panel => system => integrity check
- Try to do reindex: admin panel => system => advanced => reindexation
If it does not helps then you can get more information into atlassian-jira.log:
- for server version you can get it <jira-home>/Appliication Data/JIRA/log
- for cloud version you should make support request to get it