Most likely a brake problem (usally a tachometer) that means that the brake has been disconnected. The brakes are applied automatically during the retraction sequence to stop them rotating, if the brake is disconnected then you have to leave the gear down for 2 minutes after take-off to allow it to stop spinning. Quite why it's considered such a problem to have them rotating in the wheel well I don't know. Take-off and landing performance are both reduced via a table with one brake inop, and obviously if you have an engine failure you retract it anyway.