If DB2 was processing a very long running transaction (say updating millions of rows) and for some reason like power failure, server goes down.
When you start the server and activate the database, it may take a long time as it needs to do the crash recovery.
This is the simple script that I used on Linux to monitor the crash recovery.
db2 list utilities show detail | grep -E "Total Work|Completed Work|Estimated|Phase"
Run this script from your shell using watch command.
$ watch mon_crash_recovery
The watch script shows the output at every 2 seconds and that is the way you can see how the crash recovery is going on.