G'day Kate,
You didn't mention which model of Garmin you had. So here are the Garmin resets I have on file.
You will likely lose all data. It's always a good idea to make sure you have the lastest firmware for the unit as they "frequently" update it on the Garmin website.
Garmin Forerunner 101/201/301 Soft Reset instructions
Turn the unit off. If you cannot turn it off, due to a "lock up", leave the system the drain the battery.
With the unit "OFF"
Press and keep holding the "MODE" button
Press and keep holding the "POWER" button
Release the "POWER" button
Release the "MODE" button
The unit will ask if you are sure you want to perform a full reset (you will loose all stored data)
Reply by pressing "ENTER" button.
The unit has now been reset. You now need to enter your Garmin Forerunner personal data.
Garmin Forerunner 205 and 305 reset
System Reset - resetting a Garmin Forerunner 205/305
For Garmin Forerunner 101/201/301 reset, please click the link
This procedure should be tried before returning a unit to BHIP Ltd. (garmin GPS / HRM returns procedure)
Also, please ensure you are running the very latest version of unit software available. Check this at
www.garmin.com, then outdoor products, then forerunner 205/305, then downloads and upgrades.
"To power the unit back on, press the "Mode" and "Reset" buttons at the same time. Let go of those two buttons, press the power button, and the unit will turn back on."
If the unit is no longer displaying maps or lap data in the PC downloaded history ....perform a reset (as above) AFTER deleting all the Garmin HISTORY. This is done on the unit by navigating through HISTORY to "DELETE ALL" ---> now perform a soft reset.
Peace
Peter