Although you say you've pressed on the throttle butterfly (whoops!) I'd agree with Operation Alfa's earlier diagnosis, sounds like poor earth.
Having worked on the car recently its going to be the battery you want to check rather than the earth points on the car / chassis as you're likely to have had the battery off.
Unless you're 110% satisfied take a piece or wire wool, fine sand paper (worked for me recently) or similar and give the inside of the [negative] battery connector and the battery terminal itself a good clean up. Spray with contact cleaner once you've actually physically cleaned it and see if that works.
Its good maintenance whatever and easy to rule out.
For the throttle body - you can take off the inlet pipe, flick the ignition on and without starting the engine press your throttle and you should get a small amount of movement from the butterfly. only maybe 10% or so, its not going to open the full 90 deg but you can see if it is actually moving at all of if you've bent something and got it stuck.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso