Hello, kully sahota.
I won't worry too much if it just happens once for a long while. Otherwise, or if it follows a certain pattern, you should watch the stress on the resources of the system. System is busy (and becoming unstable) may happen for some cases when heavy swapping in and out of memory is on the way, but one hour still nothing indicates the system gets lost in the process. (There is a natural factor of random fluctuation for that.)
I won't bother you with the advice of scandisk/defrag/system resources etc. which may improve performance. But, however the system is fragmented, it only slow it down a bit, but not to make it getting lost in the process which the system is designed and good for .
regards - tsuji