@tomleslie Thanks for your answer! 

I use Maple 2017. I installed program again and there is no error any more. 

I have no idea why this happened. 

Thank you again!) 

@Rouben Rostamian  Thank you so much for your answer! It is really interesting and usefull! 

@tomleslie 

Thank you for you answer! 

Unfortuantely, i recieved another error(( Used this code:

restart;
pde := diff(u(x, t), t, t) = diff(u(x, t), x, x)+5*sin(3*x);
bc := u(0, t) = 0, u(Pi, t) = 0;
ic := u(x, 0) = 0, (D[2](u))(x, 0) = 1;
sol1 := pdsolve({bc, ic, pde});

sol := pdsolve(pde, {bc, ic}, numeric); sol:-plot3d(x = 0 .. 1, t = 0 .. 1)

And the error is following:

Error, (in PDEAdvisor/2nd_order/Series/WithSourceTerm) invalid input: rhs expects 1 argument, but received 2
 

Error is still after initial conditials. Maybe something wrong with maple?

@Rouben Rostamian  

I don't need to use diff_table) I tried to use your code too but still Maple shows an error. 
Code that I put in:

restart;
pde := diff(u(x, t), t, t) = diff(u(x, t), x, x)+5*sin(3*x);
bc := u(0, t) = 0, u(Pi, t) = 0;
ic := u(x, 0) = 0, (D[2](u))(x, 0) = 1;
sol1 := pdsolve(pde, {bc, ic});

sol := pdsolve(pde, {bc, ic}, numeric);
sol:-plot3d(x = 0 .. 1, t = 0 .. 1)

After initial conditions I recieve an error:

Error, (in pdsolve/sys) too many arguments; some or all of the following are wrong: [{u(x, t)}, {u(0, t) = 0, u(Pi, t) = 0, u(x, 0) = 0, (D[2](u))(x, 0) = 1}]
 

@tomleslie 

i'm trying to use fixed code, but unforunately it shows other mistake. This is what I put in:

restart;
with(PDEtools);
U := diff_table(u(x, t));
pde := U[t, t] = U[x, x]+5*sin(3*x);
bc1 := eval(U[], x = 0) = 0; bc2 := eval(U[], x = Pi) = 0;
ic1 := eval(U[], t = 0) = 0; ic2 := convert(eval(U[t], t = 0), D) = 1;
sys := [pde, bc1, bc2, ic1, ic2];

pds1 := pdsolve(sys);

pds2 := pdsolve(sys[1], sys[2 .. 5], numeric);
pds2:-plot3d(x = 0 .. 1, t = 0 .. 1)

Maple shows the system but after that gives following error: 

Error, (in PDEAdvisor/2nd_order/Series/WithSourceTerm) invalid input: rhs expects 1 argument, but received 2

After that it gives numerical solution. 

For me it is not important to use diff_table. I just don't know what else to use. For me it is important to show that Maple can't give an analytic solution. In a perfect situation I need to understand why)