Silus died during a Dawnguard quest because Serana killed him. I've tried searching his body, but it had nothing on it.
Mehrunes' Razor is a reward for the quest Pieces of the Past, which may only be started by interacting with Silus. In Skyrim, when named characters die, they do not respawn.
If you are playing on a console- you're never going to start the quest, much less finish it.
If you are playing on pc- go to Silus' home in Dawnstar and type the following into the console (press the tilde [~] key to open the console):
player.placeatme 240CC 1
This code will spawn a new Silus at your location.
Alternatively, you can code Mehrunes' Razor into your inventory directly:
player.additem 240D2 1
You have to finish Pieces of the Past in favor of Mehrunes Dagon to get Mehrunes' Razor through regular means. This means killing Silus at a certain part of the quest. If you're not playing on the PC, you'll have to load an earlier save before Silus was killed.
If playing on the PC, you can use the console command,
player.additem 240d2 1 to add Mehrunes' Razor to your inventory.
You can also try reviving Silus, although I am unsure if this will allow the quest to start normally. Use the console commands
prid 240CE and then
resurrect 1. Silus should go back to life after entering those commands correctly. If this does not work in starting the quest you might have to try console commands to advance / reset the quest to its correct stage. See the linked UESP wiki articles on console commands and quest stages of the Pieces of the Past quest for more info.