So I just came back after a 5 month absence to IRC. Logged on but it seems that I cannot auth myself anymore. I'm sure it's the right accountname and password since I had it in perform and it worked before my absence. But now it says username or password incorrect.

Is there some kind of way to retrieve your password?

You'll have to ask your network admins (often in a channel such as #help).
Seems possible your account was removed due to inactivity, but again, your network admins will know more.

The majority of networks that allow registering your nick will automatically remove the registration after 30 days of inactivity. Some may give you 60 days, but few will give you 5 months or more.

Just register again and you'll be fine.

depend if your using any other addon
if you open your mirc folder open system folder
and if there is a vars.ini file open it and edit it very carefully your password and nick will be listed there amongest the other scripting
but most likely the server droped you. you have been away to long just reregister--> /ns register passyouwant emailaddress and if you have a group option for AFK. To group - switch to your afk nick - Type /NS GROUP yournick password
then in the preform box put /msg nickserv identify passwordhere
then your all set