just type in command prompt "net user" alone and click enter... it wil show all the accounts that are active... chek if the system admin account is available or not...
if it doesn't show up....
ten in command prompt type "lusrmgr.msc" and press enter... local user groups will be displayed... check for th admin account and check its properties whether its active or not.. do the necessary changes and try loggin in...
let me knw if there is any issues....
all d best