Is everything else normal? speedometer, Odometer, dash lights, turn signal indication on the dash, fuel gauge, temp gauge, no MIL...?
Has any electrical work been done on it? ...like trying to tap into speed signal, or connecting an auxiliary power plug using the the Honda connector under the left pocket?
I think MPG is calculated from Vehicle Speed (Vs sensor) and Fuel Injection Pulse signals. I see 5 connectors where the signals could be getting lost between the ECM and Combination Meter. 2 connectors at the ECM, a 10 pin & 12 pin connector under the left pocket, and the 20 pin connector at the the Combination Meter. Try disconnecting, cleaning and reconnecting the connectors. Disclaimer: Some body plastic disassembly required.
Once MPG is reset, ride a short ways for it to start reading again.