Want to exit sub if range have no data

May be this........

Sub Haroon2015_RangeEditing()
Sheets("ENTRY").Select
If WorksheetFunction.Count(Range("C10:C300")) = 0 Then Exit Sub
Range("C10:C300").SpecialCells(xlCellTypeConstants).Select
Dim rngCell As Range
Dim varAns
'For Each rngCell In Range("C10:C25")
For Each rngCell In Selection
  varAns = InputBox(prompt:="Edit cell if wanted", Title:="Edit cell " & rngCell.Address(0, 0), Default:=rngCell.Value)
  If Not varAns = "" Then rngCell.Value = varAns
Next rngCell
End Sub

Haroon ! do you have only Numbers in your specified range Or You have Text Also!

@haroon

If you have text only in the range then change the code to this........

If WorksheetFunction.CountA(Range("C10:C300")) = 0 Then Exit Sub

@hemesh

Correct point raised. If the range may contain text in that case he may change COUNT to COUNTA in the code.