As @HandyHowie suggests in his comment, this appears to be a simple connection issue which is only part time. You have disconnected/reconnected at the battery, but I'll bet your problems lie at the other end of these cables ... most likely on the ground side, but you'll be better off pulling it off of both ends (positive and negative) to ensure you have a complete connection and there's nothing blocking your connections (like corrosion).
My suggestion is to pull first the negative ground from the opposite end of the battery (where it connects to the vehicle), clean up both sides of the connector with 200-300 grit sand paper (or a good stiff wire brush). Leave this off (not connected for the time being). Clean the spot where the ground is attached to the vehicle using the same method. This could be on the base of the alternator, the engine block, or the actual body of the vehicle. Make sure the bolt (or whatever attaches the cable) is completely clean. Check the starter connection and make sure it's tight (I doubt there is an issue on the positive side, but this will ensure there's no issues). Also check the connection at the alternator to ensure it's tight as well. Now reinstall the negative ground back to its mounting spot. Check your battery clamp connectors to ensure they are clean and serviceable. If these are clean, coat them with some grease to help prevent corrosion in the future. Now check out to see if you have lights and are able to start.
When you check your connections at the battery, ensure they are tight and will not move. If you are able to move them with moderate force, you'll need to figure out what is keeping them from becoming tight and fixing the issue.
You also need to check the entire length of the cables (both battery cables) and see if there is any bulging in them. If there is, this means that corrosion is occurring inside the wires and will require replacement. The corrosion creates a huge pocket of resistance which can cause issues with getting your vehicle started.