Well if this TIU is as old as I suspect, it is possible that it just up and died.
A couple of things you can check:
1) when the TIU powers on, there is a red LED light that will Flash 1 to 5 times to indicate what address is assigned to the TIU. If it flashed more than 5 times, that's bad news.
2) What revision is the TIU? There is a label on the bottom It may read H, I, or L. If there is no sticker it is the first revsion of TIU which is G. Again these not the best TIU's only made for a couple years before the much better version H came out. the Rev. L is latest version available now and by far a vast improvement over the G.
3) Lastly, it could be something as simple as a loose tranciever board in the TIU or the Remote. Use an old school curly telephone cord to and connect your remote directly to the TIU (REMOTE port). If your TIU begins to respond normally, you may only need to reseat the tranciever board in the TIU or the Remote (the more likely culprit).