This tutorial provides the steps for hiding and unhiding rows in Google Spreadsheet. You can hide rows in Google Spreadsheet by highlighting the row and selecting Hide Row menu option from right click context menu.
A hidden row in Google Spreadsheet will display an indicator as shown below. Clicking on the indicator would unhide the row.
If you freeze the rows after hiding then the unhide indicator will be disabled and you will not be able use it until the rows are unfreezed.
Similarly if you try to hide frozen rows then Google Spreadsheet will display an error message that “You can’t hide rows in the frozen header section”