1000 reasons, you must inspect your system step by step. It can be dead battery (if PRM went down slowly), it can be overheating (usually brake does not work before landing in such case), could be malfunction of timer, but that is usually permanent error and if it worked after that, it is not probable, it can ESC on way to heaven if you heard problematic run before (loosing synchronization) and it will also make troubles afterwars, it could be connectors of timer, if you can simply restart it after landing like when you connect battery, it could be disconnected motor <-> timer wire (if it is KR timer) which will cut off power in case of stopped motor, but most usual in succh case (sometimes work, sometimes not and accidentally stops) is problem with battery connector - that was also case on WCh with model of Samantha ... exactly like that.