The Hebrew text literally translates as his hands became weak.
When Ish-bosheth heard news that his general had been killed, he was distraught. There was no one now to unite the armies of Israel and protect him from David. The death of Abner signalled the end to any military power which Ish-bosheth had.1
1 When Ish-bosheth, Saul’s son, heard that Abner had died at Hebron, his courage failed, and all Israel was dismayed.