[help]Network connected but cannot ping everywhere

Pls somedy help me..my office have switch 24 port,and suddenly all the computer is cannot connect anywhere,every computer status is connected,and than I try to PING to one and another computer but it show "destinantion host unreachable" or " request time out"
fors first time I think my switch is broken and I decide to use another new switch,it can work but after using it half an hour it cannot connect again,the same problem from before

how do I solve this problem? What is the cause of this problem??

thxx..