Once an STP loop has been identified it is important to isolate the reason
behind the loop. One of the first things to check is to verify if the Spanning
Tree Protocol is running on each of the switches. A switch should only have STP
disabled if it is not part of a physically looped topology
The IOS
command to verify STP operation is:

Switch#show spanning-tree
It
will be clear from the output of the show spanning-tree
command if STP is not running.

It
is important to remember that spanning tree operates on a per-vlan basis by
default on Cisco devices (PVST/PVST+) and executing a show
spanning-tree command will report on VLAN 1 only. Use the
show spanning-tree [vlan ID] command to verify STP
operations for other VLANs.
Should STP not be operating, it can be
enabled using the spanning-tree vlan ID command.