How strange.

Normally I use IRC and /s -m blablabla and /j room work just fine.. However, today I open up IRC and I keep getting weird erros.

S Unknown command
-
J Unknown command
-

Did I accidentally change some setting or something, or did my mIRC go crazy?

sounds like your aliases are gone. open your scripts editor to "Aliases" and enter this if it is not there:

/op /mode # +ooo $$1 $2 $3
/dop /mode # -ooo $$1 $2 $3
/j /join #$$1 $2-
/p /part #
/n /names #$$1
/w /whois $$1
/k /kick # $$1 $2-
/q /query $$1
/send /dcc send $1 $2
/chat /dcc chat $1
/ping /ctcp $$1 ping
/s /server $$1-