It often happens after your restart, device is unstable, device replacement, or after an outage, the IPSec tunnel will stop working.
Do not panic, you can restart the VPN session.
To restart or reset the VPN session you can issue this command:
clear ipsec sa peer [your peer IP address]
or, you can issue this command to clear all tunnels:
clear crypto isakmp
From my experience, you have to issue this command from both side A and B.
Last, to check the VPN tunnel status you can issue the command:
sh isakmp sa
If there is any traffic that requires the VPN tunnel, the connection will establish itself again.