I had a simple 5V relay that works just fine with arduino. A few weeks ago it stopped working. Do you have any idea why? I only use the 5V power line on the arduino, so I don't think it "overcurrented".
Is the Arduino still working?
Yes, the arduino works completely fine. My circuit to test the relay is just coil pin 1 connected to 5V and coil pin 2 connected to ground.
Show us a good schematic of your circuit.
Show us a good image of your ‘actual’ wiring.
Give links to components.
Posting images:
https://forum.arduino.cc/index.php?topic=519037.0
My circuit as described above is only just wires connected to a relays coil going to ground and 5V. The component is a relay that I got from this kit : https://www.amazon.com/Adeept-Ultimate-Mega2560-Learning-Guidebook/dp/B01ITBIYJK/ref=sr_1_1?dchild=1&keywords=adeept+mega&qid=1590791173&sr=8-1.
Please realize we cannot see what you have in front of you.
We don’t have that kit so it is quite difficult to diagnose what is invisible to us.
It could be a broken wire.
It could be if the 5v power is taken from the Arduino the on board regulator has been damaged.
Maybe there is an over temperature situation.
The relay maybe drawing enough current to be at the boarder line point where the supply cannot provide enough.
It could be the relay coil is intermittent.
It could be the driver circuit for the relay has a bad connection or component.
It could be a cold solder joint.
It can be hundreds of different things.
We can guess but that is not a realistic way for us to help you.
Help us help you, supply information when requested.
What resistance do you measure across the coil of the relay?
Thank you for the supposed problems. My circuit as described before, is really simple and I don't think it would bring much value to draw it. It is just two wires connected to the coil. No driver at all. Board seems fine and I changed out wires to be sure. Something to note, relay was working fine with 5V until a couple of weeks ago. It still works fine but only works with a 9V battery now.
About 71 ohms.
It is just two wires connected to the coil. No driver at all.
...and No diode at all?
...and the Arduino pin still works?
...and the relay didn't survive?
Impressive! (could've been worse)
Luckily a single relay board c/w driver can be had for about $1.
Yeah, luckily its cheap. I gaven up now. My relay stopped working on a circuit with a back emf protection diode. Strange, not sure what did it, but at least relays are cheap. Thank you all for your help.
dlloyd:
Luckily a single relay board c/w driver can be had for about $1.
Used to be about $1.
Since Covid, has gone up by 40 to 100% along with everything else!
Absolute cheapest now $US1.39
Actually not too bad for a relay, but many other things have become ridiculous! 
Actually not too bad for a relay, but many other things have become ridiculous!
Yeah ... couldn't get the nerve to check. My first and rather large purchase from AliExpre$$ (pre-covid) could now be a write-off, I guess.
First, you can test the relay without a microcontroller.
- You have 2 sides of the module:
a) Input side: at least 3 pins, Vcc, GND and input signal.
b) Controlled output side: COM, NO and NC. - To test it without mcu, connect a 5V and GND at input side
- Connect a wire to GND line and then touch the input signal pin, you should hear the clicking sound + the activation red LED at input side should be on if your module has one.
That appears to be a very simple relay. If it operates with 9v but not with 5v, it may be sticking inside. Have you tried using a known good 5v-6v power supply to see if it operates? A lot of problems often come down to an inadequate power source or wiring issues.