With some routers/switches it can happen that - e.g. due to hardware changes - the port number of an interface changes. In order for PRTG to continue monitoring the same interface the port number must be changed.
PRTG has a built-in auto-recovery feature that tries to find the new port number automatically, but this kind-of-fuzzy-logic won't always work.
There is no GUI functionality in PRTG's interface to do so, but you can edit the .prtg file (it's XML) manually.
Here are the steps:
<sensor gid="{69FADE87-866A-4C94-AEBB-B293292DB508}">
<name>
YOUR NEW SENSORNAME (e.g. Port WLAN-1 on firewall (10.0.0.1))
</name>
<iface>
14
</iface>
Now both the new and the old sensor should be monitoring the same interface.