|[Note: I wrote the material below in response to an older post on the subject noted. This site is forcing me to post it as a new question. It may be helpful to some.]|
I had a similar problem (no wireless printing after it had been working for years) after my computer and printer had been unplugged for a week while I was on vacation. Here is what I did to bring back wireless printing based on combining info on the Internet and trial and error.
Computer is XP Home SP3.
I uninstalled and reinstalled all the printer, as well as PaperPort, software. This did not help (but may have been necessary).
Then I created a new port for wireless printing, as follows. Select Brother MFC-7820N Printer (not the "USB" version of this printer) in Start > Settings > Printers and Faxes. Right-click > Properties > Ports tab. The existing checked port, for whatever reason, had the name "BRN_C91457" (it was correctly a Standard TCP/IP port). I decided to create a new Standard TCP/IP port.
Add Port button > select Standard TCP/IP Port > New Port button. In the next dialog, I entered the IP address 192.168.2.100 in the first box; the second box fills in automatically. (I obtained this IP address from My Computer > Brother MFC-7820N LAN > Properties > Network Setting tab. Go here first; don't just use 192.168.2.100 randomly without verifying that it is correct on your network.) Then press Next button. The rest is taken care of automatically. The new port is named IP_192.168.2.100; make sure it is checked and the BRN_C91457 port is unchecked. Press Apply and Close. As a result, printing wireless worked again.