How to enable day light cycle?

Since this is your server, you probably have access to the console. If you do, simply type gamerule doDaylightCycle true.

Another way is to edit the level.dat file of your world. You are going to need a piece of software like NBTExplorer or Universal Minecraft Editor to do that. Download the level.dat file from your server (the file is in server_dir/world) to your pc and open it in the software of your choosing. Expand level.dat, expand data, expand GameRules and change the doDaylightCycle field to true. Save the file and upload it back to your server. The game rule should update after you restart the server.