The previous owner of a

Question

The previous owner of a computer I purchased was the administrator. I signed in as a "guest"....It worked fine for 2 weeks,then something happened and I am not able to use this computer....
It wants the administrators password. The person has moved out of state....
Am I out of luck????
I am now using my other on-line computer to get thru to you to solve the issue....

Can I become the administrator instead of a guest ????

Can you help ????