Transformers and Electric power Ltspice
Since your simulation schematic appears to have the transformer connected as a Flyback coupled inductor, I'm going to assume that you're trying to implement a Flyback converter. But, there are lots of problems with the circuit. 

First, \$D_5\$ is directly across \$L_1\$. That's a problem because of the Volt-seconds it forces on \$L_1\$. When \$M_1\$ is conducting, \$L_1\$ has 2000V across it for time \$T_{\text{on}}\$. But when \$M_1\$ is turned off, \$L_1\$ can only have the forward voltage of \$D_5\$ (\$V_{\text{D5-forward}}\$) across it for time \$T_{\text{off}}\$. Let's say that \$V_{\text{D5-forward}}\$ is 4V (that's probably about right for a 6000V diode). That means that \$T_{\text{off}}\$ has to be 500 times \$T_{\text{on}}\$. Otherwise, in a real circuit \$L_1\$ would saturate. But, it's hard to say what a simulator will do here, since often this kind of thing isn't well modeled. This alone could cause your problem getting any kind of sensible results. If you want \$T_{\text{on}}\$ to be somewhere close to the same as \$T_{\text{off}}\$, then \$D_5\$ doesn't belong across \$L_1\$ at all. Of course, \$M_1\$ will have to withstand some multiple of 2000V. 

Second, for a Flyback, \$D_2\$, \$D_3\$, and \$D_4\$ don't belong in the circuit either. Only \$D_1\$ would be used as rectifier in the output. When \$M_1\$ conducts, energy is stored in \$L_1\$. When \$M_1\$ is turned off that stored energy is dumped into the output load through \$D_1\$. So, \$L_2\$ dot would connect to \$D_{\text{1-anode}}\$, and \$L_2\$ non-dot connects to ground.