If you are using Pro, there is/was a PC on your network that does the licenseadmin for the PC's running Pro. This can be any machine, it doesn't have to be a windows server.
You should try to find this machine because it is the one with all of the sequence numbers for the users as well as the licenses that enable those sequences.
One way to try to find it is to go to one of the client PC's with pro installed and in start menu under ICE.TCP Pro the configwizard entry will point to the original license server. You can usually see that by a mouseover or by right clicking on it and doing properties and looking at the shortcut.
If you find that server, run licenseadmin on it and you should be able to see the users.
If not, you will need to designate a new server. Get your licenses together and install the admin portion onto it. After entering your licenses into licenseadmin, run configwizard ON EACH CLIENT MACHINE by BROWSING to the new server share TCP5_USER and then into TCP-Pro and run configwizard to reget the sequence number.
Once you start this process you will have to continue until configwizard has run on all of the client PC's.
There are several other posts in this forum that have more detail on this process if you need it. Also, be SURE the you have a version >=5.0.81 if you will be installing on any win 2k or xp machines (see also other posts on this topic).