All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, we praise Allaah who guided you to Islam and enabled you to repent before it is too late. You should know that there are some conditions that must be fulfilled so that your repentance could be accepted, like not committing the sin again, regretting having committed it, and being determined not to return to it again. If the sin is committed against a human being, then it is a condition to return the right to its owner; for more benefit, please refer to Fatwa 86527.
Therefore, you should endeavour to return this laptop to its owner or to his heirs if he died, and you have to return the amount of rent of an equivalent laptop throughout the time it had stayed with you. Then if you had made every effort to reach its owner and you were not able to, you should give it in charity along with what you are obliged to pay in return for its use on behalf of its owner, as clarified in Fatwa 89762.
Indeed, you may ask the experienced people in this matter to estimate the amount you have to pay in return for using it.
Finally, you should endeavour to perform good deeds, like giving charity, as this makes your repentance more likely to be accepted and rectifies your heart.
Allaah Knows best.