|This a Linux OS? I ask so as to clarify that as generally this problem comes for Windows OS?|
Regardless, if you do a google trawl/search using the string/words:
Reset admin password Linux
Recover admin password Linux
You will get a series of hits detailing how to do it.
If your OS is Windows based then replace Linux in the above with windows "x" - where "x" is version of Windows involved.