I assume the input to your comparator is isolated from the AC? i.e. From the secondary of a transformer?
But so far the i have tried using basic zero crossing circuits like comparator comparing the sine wave with a ground but to no avail.
That's how I did it... Is the comparator "firing"? (Is the output changing state?)
If the comparator is running from a single supply, it might not work near ground. If the comparator is running from positive & negative supplies, it should work fine and put-out a square wave.
If you have a single supply, you can set the reference voltage higher, and calculate when the actual zero--crossing occurs.
Actually, it's probably better if you set the comparator reference to around 45 degrees (half the peak AC voltage). The actual-exact zero-crossing is difficult to detect. If your detection circuit is detecting small voltages, a little noise on the AC line near the true zero-crossing can give you a false-trigger. The peak is even more difficult, since the AC voltage vaires a bit. Somewhere in-between is more ideal.