Just replaced a new ecu on it and wont start. how would i reset it

Well, ya might wanna do the Math; the money you spent for the New ECU, Plus reprogramming, Plus a new Valve timing belt, Plus other issues which are not evident because, the Car isn't running... I'm pretty sure we've past the Car's value.
https://www.nadaguides.com/Cars/1998/Hyundai/Tiburon/2-Door-Coupe/Values
However,
You need the Key that is for that ECU
OR
A *new key needs to be programed into the ECU
Also, since the ECU has been unplugged for x amount of time; The ECU needs to be programed, Now
*Your OLD Key may not be readable.

For do it yourselfers; When replacing PCM/ECUs,
Body PCMs, Programable Fuel injection, Instruments and such... with used parts
Recommend; getting access to a complete RUNNING donner Car at Pick and Pull or Pull Part or...


Sorry, I'd like to help but, Seems to me a Dealer only problem at this point.

You can't program it unless you have a factory scan tool , this is dealer only .

sounds like you have a failing ecu. the capacitors in the ecu are worn out. there has been a lot of problems with the ecu on mazda's over the years, only take one out put overload from the alternator and the ecu starts playing up. no codes come up as it don't code its self. having the battery off is like a reset.

Check connections to ECU under pax seat I believe, possibly a failed ECU, make sure you buy a new coil as well if replacing ECU.

Reset the timing and check starter ot see if working. Verify fuel pump is actually pumping fuel.

ecu can be reset by disconnecting battery. this is likely not your problem..

yes there is; remove the positive cable from battery to reset the computer before you start the car. leave off for 5mins to let computer adapt the new ecu. hope this helps!!!

next time it wont start, pour hot water over ecu, if it starts replace ecu, you will need a scanner to reset as it will go into theft mode when you fit new ecu, beware that this is a common fault and you can buy 2nd hand faulty computers.

check battery and fuses

Reset the ECU? Who told you to do that? Check the basics. Does it have fuel and spark while cranking? Did those wires get put on correctly when they were replaced? Be sure the wires are correct. You can go here and pick the right engine to see how the wires should be run:

http://autorepair.about.com/library/firing_orders/bl-eagle-firing-95.htm

You didn't mention which engine you had (with or without turbo).