I believe this question pertains to a specific server where the problem is related to a nether portal being trapped intentionally within player-claimed land. This is done to prevent players from entering bases they are not supposed to be able to access. Because you are unable to re-enter the portal without leaving it first (not possible in this scenario), the only way out is by teleporting.
If you have a home base, you can try to contact a server administrator to locate your home base via CoreProtect logs, if that plugin is installed. If you have set your home via /sethome, use /home to visit your sethome. The commands I just suggested are from the Essentials plugin which is present on most multiplayer servers.
If you don't have a home base, and you were just exploring and happened to enter a portal to the nether, you can probably reach the server spawn by entering /spawn or by visiting a server warp.