I updated to the lasted version mIRC 7.44 and have found some of my scripts no longer work. I have been using miRC for many years and never in the past have had any of my scripts failed to work.
mIRC works in the same way as all applications that support scripts/add-ons, eg. like Firefox and Google Chrome. Very often, when a new version of these browsers is released, an add-on will stop working until it is updated by the author of that add-on. The same applies to mIRC scripts/add-ons. However, as you have discovered, with mIRC you normally do not need to update your scripts/add-ons. That is because updates to the mIRC scripting language are usually designed to be as backward compatible as possible.
If you have been using mIRC for years and have upgraded many times and have never needed to update any of your scripts, that is actually pretty good going.
However, if one of your scripts has stopped working, you will either need to contact the author of the script/add-on to ask them if there is an update, or you can try updating the script yourself - but you will need to have some idea of how to script to do this.
First, you will need to find out which part of your script is causing the issue. For example, if you usually type a /command and it has stopped working, the issue may be related to that /command and you will need to search for the /command definition in your scripts and then add debugging /echo commands to that command to see where it stops working.
If you can narrow it down to a particular /command, you can try posting the /command definition (not the whole script - that will rarely fit in a post) to this thread and someone on the forums may be able to help.
This will help in tracking down the cause of the issue and it may turn out that either 1) you will need to update your script or 2) you have found an issue in the latest version that can be fixed in the next version.