Tenant says there is very minimal hot water in upstairs unit

[Samantha Klein]

I have an upstairs unit in a duplex and my tenant just texted and said there is no hot water, she said it gets slightly warm but that's it, there is a separate water heater under the cabinet in the kitchen next to the bathroom, what could the problem possibly be? She's been in the unit for 7 months and have had no issues at all until now. I'm having my handyman check it out later but does anybody have an idea?