306 will not start and all electrics have gone

Hi everyone Please help with my problem. This is gonna be quite long so bear with me. I have a 1997 306lx 1.4i. I took it to my local tyre and exhaust garage for a new middle box and 2 new front tyres. I left it with them for the day. I go to pick it up. The guy says to me that they had problems starting it prior to putting it up on the hoist. But they did manage to start it.I pay them for the job that was done. I get in my car. Both front windows were left down by the mechanics. Went to start the car. I noticed that the temp gauge needle was as far over to the right as it could possibly go. The digital clock was flashing. Upon turning the key the car turned over fine but wouldnt fire up. At that point i heard a buzzing from one of the relays in the fuse box. The dials went nuts.Speedo went to 100mph, rev to 6000, fuel gauge to 3/4. temp gauge went back to normal position. I realised that i had a big problem. I wanted to secure the car but found that i could not wind the windows up. Motors just made noise but did nothing. Went back to the garage to ask what they did to it as it was fine when dropped off that morning. They said that they had trouble starting it. So they left it for 5 minutes then returned and it started. The guy also said that the fuel gauge was showing nil. I had over half a tank when i dropped it off. He thought that the gauge was faulty. Anyway i got a tow home. Checked battery terminals.They're fine. Charged the battery just in case it was a little low. Battery charged up fine. Tried once more to start it up. Same scenario again. Still no functioning windows, mirrors, radio, clock, gauges, or locking. I have had to cover the windows with plastic sheeting for now. I am very dubious about going to a dealer with this problem as i think they will overcharge me and probably do a process of elimination with parts in the hope of fixing the problem when it could just be a simple thing like a dodgy relay, fuse or whatever. I was wondering whether anyone else has had a similar problem or if someone can give me any help in locating typical problem areas with a fault like or similar to this. Thanks for your time and effort in reading this article. I personally think im fooked and this is gonna cost me more than the car is worth!!