All VPS servers include atleast 2 Terminal Service sessions and if you simply disconnect from your terminal services session or leave a session open on another computer you may try to log into your VPS and get the error message that all terminal service sessions are busy. The way around this is to log in via the console terminal services session and reset, logoff or close the idle terminal service sessions so you can connect. To connect to the console port you'll type the command:
mstsc /console /v IPADDRESS.
Where IPADDRESS is the IP address of the VPS you're trying to connect to. Once connected you'll be able to work on the VPS as if you had a keyboard, mouse and monitor connected to your VPS as you would with a physical machine.
If you need more than two concurrent users on your server please contact email@example.com for pricing for additional user licenses.