I recently had a loss too. It turned out to be some corrosion on the panel connections. I have kyocera panels they are a watertight box, but the holes you punch through must also be watertight or it is worthless.
First, throw a blanket over your panels and block ALL the light. I usually throw a few heavy things on top of that to make sure I don't get bit.
Disconnect the charge controller. I have a switch. Not sure what you have.
Open the box and pull a reading on the output. It should be zero. Next, polish up all your terminals. I usually use some emory cloth 220 wet/dry paper. Remember to clean both sides of the Ring terms and the terminals on the panels where they connect.
Next, just to make sure, I took the blanket off and took an OCV (Open Circut Voltage) across the panels (basically where your two ring terms connect). You need to read your panels theoretical output, but it is likely close to 17-20v. THe open voltage should be pasted to the bottom of your panel. If the panels are not putting out, you either have a bad panel or a bad blocking diode. The blocking diodes on my panels are inside the box so you can jump around them if you want to (I do not). If that is the case, check the open side of the diodes for open voltage. You may see some voltage gain if you don't run through the diodes. If you still have no voltage, time to buy new panel(s) though I would be curious why...
If you are getting the proper voltage, re-cover each panel with the blanket and coat each side of the ring terms and the connections with dielectric grease and tightly connect them. I wouldn't use any connection outside without using dielectric, but that is just me. I also prefer heat shrinking everything, but you may not want to.
Take everything off and see what you are putting out. If you still are not getting the correct V/A, you know the problem is after the panels. I am assuming you looked for a fuse to start off with, so assuming that is ok, I wuold go to the charge controller and take an open voltage reading. Depending on how you wired your panels, you should see no less than a panel puts out to all of them in series. Again, without knowing how they are wired, hard to say. Also, if you did not properly size your wiring, you will see V loss there, but should still work.
If you are getting the right voltage there, you know the problem is after that (charge controller - batteries). Make sure your charge controller is putting out the proper voltage to your batteries. Also check the connections to your batteries and see what they are receiving. If you are getting everything to your battery terms that it is rated for, you know the problem lies with your batteries.
Anyways, it is a process to figure these things out. I suspect your problem lies in corrosion at the panels as that is the most common issue. If you are getting corrosion there, you have not properly installed your panels. The dielectric will help, but you need to run water tight conduit and connections between the panels and to the boat. I used LeakTight conduit and connectors from Home Depot. $2/connector and probably $15-20 for the conduit. Make sure to get the all plastic ones and avoid the crappy galvanic metal rings as they never seem to last in a marine environment.
My opinions!!!
Brian