Hello,
It sounds like your worksheet has become protected, try first to remove protection:
- On the ribbon, click Review.
- Click Unprotect Sheet. If the sheet was protected with a password, you'll be prompted to enter it. Otherwise protection will be removed immediately.
Once the sheet is unprotected, select the cells you want bordered and use Home > Borders to apply the style you need. If you still can't draw borders after unprotecting, right click a bordered cell, choose Format Cells, go to the Border tab, make sure an appropriate color and line style are selected, then click the diagram edges where you want borders applied and hit OK.
Let me know if this works!