I can not get mainwindow handle id windows ce

You could probably use SetForegroundWindow function | Microsoft Docs[^] to bring the window to topmost

Something like

[System.Runtime.InteropServices.DllImport("user32.dll")]
internal static extern int SetForegroundWindow(IntPtr hwnd);

...

void YourMethod() {
   ProcessInfo[] procs = null;
   if (denizbankProcess == null || (denizProcess != null && denizProcess.HasExited))
   {
      procs = ProcessCE.GetProcesses();
      if (procs.Where(p => p.FullPath.Contains("DotNetDeniz")).Count() > 0)
      {
         var pid = procs.Where(p => p.FullPath.Contains("DotNetDeniz")).First().Pid;
         denizbankProcess = Process.GetProcessById((int)pid);
         SetForegroundWindow(denizbankProcess.MainWindowHandle);
      }
      //denizProcess = Process.GetProcessById(Marshal.ReadInt32(procs.Where(p => p.FullPath.Contains("DotNetEMVDeniz")).First().Pid));
      else
      {
         var psi = new System.Diagnostics.ProcessStartInfo();
         psi.FileName = "\\PocketStore\\WIN_CE\\DotNet\\Deniz\\DotNetDeniz.exe";
         denizProcess = Process.Start(psi);
         Thread.Sleep(10000);
      }
   }
}

Note that if the window is minimized, you need to use ShowWindow function | Microsoft Docs[^] to restore or maximize the window first.