You dont get LOS means your connectivity to OLT is fine as I said earlier.
It depends if your LCO has put OLT in BSNL exchange or somewhere else. If it is in BSNL exchange only, then mostly the issue is from backend since there is chance of fiber cut. If OLT is placed in some other place and LCO took fiber up-link from exchange to his OLT site, it is mostly because of fiber(up-link) cut.