This can be caused by a few different things.
Your computer may be set to automatically download and install windows updates. This can cause the computer to reboot if it needs to without you doing anything. To change this, contact your I.T administrator.
In some cases it could be that hardware in the PC is overheating or failing. You will need to contact your I.T department for them to go through the logs on your computer to see what the problem is. From there they will either contact us for assistance or they may choose to resolve the issue themselves.