My Garmin Forerunner 45 cut out the battery went flat. I had over 15000 steps from both exercises today 27/03. The mileage I did from these 2 exercises appeared.

Steps are measured and recorded independent of the activity using the accelerometer. Also, if you want to add steps manually, that won't be possible - Can I Enter Steps Manually? There are some workarounds discussed in that thread. 

Alternatively, you can try contacting Garmin Support to see if they can do it for you, but no guarantees as they may be bound by Garmin policy.

Hello JamesBarnes952,

This won't solve the problem of adding back the missing steps, but may help steps from getting lost in the future. I learned it the hard way. My Viviactive HR often simply resets the steps counter back to zero during the day. (Glitch, I guess) Actually, when that happens, all of the day's metrics go to zero as well. 

To make sure I get to keep the day's steps so far, I use bluetooth so the phone & tracker are communicating. Next I open the Garmin Connect Mobile app. It can stay open in the background & out of sight while using the phone for other stuff.

Then, during the day, I periodically unlock the phone- even if I don't necessarily want to see Garmin Connect. Anytime the phone's unlocked, Connect will upload data from the tracker to the phone. (In my case, since I don't use data, information does not upload to Garminland until I get to WiFi.)

If one does this before the tracker loses resets itself to zero, those steps will be at Connect on the phone, so be sure to unlock once in a while.

When the tracker resets itself to zero, the Connect steps stay the same. New steps are added to the existing total on Connect. Most of the time, in an hour or so, Connect magically tells the tracker about the missing steps & other metrics that got nuked from the tracker (like bike rides, runs, calories, intensity minutes, etc.).

For Example: 

Have 8,000 steps on the tracker. Unlock the phone, the 8,000 go to the Garmin Connect Mobile app. 

Phone resets itself to zero. Connect still has the 8,0000. Tracker shows Zero steps. All is not lost...

Walk 2, 000 more steps. Tracker will show 2,000. Unlock the phone & Connect will show 10,000. (8,000 + 2,000 new)

Eventually, the tracker will also show 10,000. All of the other metrics that got sent to zero during the nuke will also update on the tracker. 

Then, to ensure nothing goes haywire if the app crashes, I'll sync the mobile Connect app to upload it Garminland.  (I don't use data, so I wait until I can use WiFi for these uploads.)

This work around is clunky, but I hope it helps you not lose your activities when the tracker wipes out your day's accomplishments.

Please Enjoy Today!

Paul