Why am I getting the error message downloading.. Do not turn off target when I turn on my phone?

[shukeem richards]

downloading.... Do not turn off target

i have the t-mobile Galaxy s4 sgh m919 and i restarted my phone and ma screen went straight to "downloading... do not turn off target" i did this because my phone was lagging, please i need some serious help

 

[Golfdriver97]

Re: downloading.... Do not turn off target

Welcome to the forums. You somehow got into download mode. Pull the battery for several minutes, and then try rebooting again.