Well, I'll tell you one very common cause of this problem: IMEI/NVRAM tweaking.
Having modified my Mediatek device's IMEI number countless times due to null IMEI, caused by flashing incompatible or corrupt custom files- my device got this problem. Luckily for me, I already made a stock backup of my device's NVRAM.
Fixing no signal in 3G only network mode
- MTK Droid Tools - Download
- Faulty Mediatek (MTK) Device (of course)