My oppo a12 has not starting after a restart?

There is some possible problems and some diffrent solutions.

Sometimes issues might be caused by the phone case that you are using to protect your phone. So, you should remove the case before starting.

Power cycle:

Your phone might be "frozen" (screen freeze), OPPO can freeze during restart so what I would recommend you to do is:

Press the power botton and hold it for 20-50 second without releasing it. Repeat this two times to be extra sure.

Charge it:

If you haven't tried this yet your battery might be the problem, plug it in to a wall and wait for 20-25 min (Don't leave the unattended for a longer time while you have this prolem) , after that try to start it.

If it doesn't turn on then, plug it in and see if the charging symbol shows up;

If Yes- Let it charge if it have low battery and wait a while then try to start it.

If NO- Let it charge for another 15 min and then try to start it.

(Check so the charging port is clean, don't use anything made out of metal doing that)

Screen:

So if none of above worked for you then it is time to check the screen since OPPO A12 uses an IPS LCD display we can easily check if the display lights up. Go into a dark room and try to start the phone check if you see the black screen get a little brighter.

If it does there might be either some hardware or software issue.

Call youself:

I would try to call the OPPO from another phone and if it ring (vibrates or make sounds) then its likely the screen.

You might need to check internal cabels that connect the screen so they arent damaged or swap screen.

Reset from a PC:

1. Connect the OPPO A12 to the computer using a USB cable.
2. Let it charge for 15 minutes.
3. Disconnect the cable from the phone (not the computer).
4. Reconnect the cable within 10 seconds of disconnecting it.
5. Charge OPPO A12 for another 30 minutes.
6. Press and hold the power button for ~5 sec.
7. Press Reset on the screen (if reset doesn’t appear, press and hold the power button for 30 more seconds)

I hope any of these above solves your problem otherwise I would recommend you to have someone to take a look at your phone, and good luck!