What errors, if any, is showing on the DP>HDMI connected Monitor?
Errors like " No Signal" or "Out of Range Signal" as an example.
If it worked when you first connected it but not later indicates some setting in Radeon Settings or your Monitor was changed when you powered off and then back on again.
If you should purchase a DP>HDMI Adapter make sure it is "Active" type where the DP signal is converted to HDMI Signal BEFORE reaching your Monitor. Unless the DP>HDMI cable is an "Active" converting cable.