[CentOS] sshd problem

my server is on centos 5.5,it is just a new reinstall system

i build a php depend on server and used nginx,php,mysql

ssh is default 22 port

The system perfomance good a few time.but it is always happen a problem only need hardware reboot to solve.the server is on idc.so i can not see the local screen information.

the appearance is: 1.it can not use ssh to connect,it show server is down or rebooting 2.ping is ok 3.http can show nginx's information(nginx/0.8.46),but can not load right page,and it show 403 forbiden page

i use another server use ssh -vv xxx.xxx.xxx.xxx,and the responce is:

SSH2_MSG_KEXINIT sent

close

sshd hangs after SSH2_MSG_KEXINIT sent

i do not know what happen,please give me a good answer

thanks erverbody