Look in your mIRC directory (the location where you installed mIRC) and see if there's a file called popups.ini
If it's there, open it with notepad and check if there's a section called [lpopup]
The default for that section looks like
[lpopup]
n2=Info:/uwho $1
n0=Whois:/whois $$1
n1=Query:/query $$1
n3=-
n4=Control
n5=.Ignore:/ignore $$1 1
n6=.Unignore:/ignore -r $$1 1
n6=.Op:/mode # +ooo $$1 $2 $3
n7=.Deop:/mode # -ooo $$1 $2 $3
n6=.Voice:/mode # +vvv $$1 $2 $3
n7=.Devoice:/mode # -vvv $$1 $2 $3
n8=.Kick:/kick # $$1
n9=.Kick (why):/kick # $$1 $$?="Reason:"
n10=.Ban:/ban $$1 2
n11=.Ban, Kick:/ban $$1 2 | /timer 1 3 /kick # $$1
n11=.Ban, Kick (why):/ban $$1 2 | /timer 1 3 /kick # $$1 $$?="Reason:"
n12=CTCP
n13=.Ping:/ctcp $$1 ping
n16=.Time:/ctcp $$1 time
n15=.Version:/ctcp $$1 version
n17=DCC
n18=.Send:/dcc send $$1
n19=.Chat:/dcc chat $$1
n20=-
n21=Slap!:/me slaps $$1 around a bit with a large trout
You can copy and paste that overtop of what is currently in that section (if anything) and your nicklist right click should work again.
You may need to restart mIRC.